Yes, Bungie is working on Halo for PC, if you count a lone developer at the studio collaborating with fans to work on releasing a new PC patch. It's keep the Halo multiplayer platform working post-GameSpy.

Hosting service GameRanger announced they'd be filling in for GameSpy as they shut down their systems, but some game's will need to be tweaked with a patch first.

Yesterday, the Steam registry showed Halo, Halo 2 and Halo 3 being upcoming content to be released on Steam, which raised a few eyebrows since Halo 3 had not publicly been developed on PC, and that Microsoft would be using rival Valve's digital distribution.